This invention relates to electronic switching circuits and more particularly to such circuits wherein the principal switching elements are thyristors connected in an inverse parallel relationship.
To control power in an AC load circuit, semiconductor switch devices have been used because of their inherent advantages over mechanical and electromechanical switching elements. A common form of AC switch comprises a pair of thyristors connected in inverse parallel fashion which requires a driver circuit for the gate terminal of each of the thyristors. There are several known methods of supplying gate drive current to inverse parallel thyristors. These methods include pulse drive, continuous drive and load current drive.
At common, relatively low AC power frequencies, the continuous drive method is preferred to avoid voltage spikes across the switch circuit which may occur with load current drive and to avoid the possible turn off of all the thyristors under light-load current conditions which may occur with pulse drive. However, at relatively high power frequencies, for example 20 kHz, voltage spikes have been found to occur across the switch circuit even with continuous gate drive delivered to the thyristors. Such voltage spikes can create electromagnetic interference and may be otherwise detrimental to associated circuitry. It is therefore desirable to construct an AC solid state switching circuit, for use in relatively high frequency AC systems, which maintains the desirable characteristics of an inverse parallel arrangement of thyristors but does not exhibit excessive voltage spikes following zero crossovers of the current waveform.